The present application relates to semiconductor devices, and more particularly to fin field effect transistors (FinFETs) having vertically oriented digital alloy channels.
The use of compressively strained silicon germanium (SiGe) alloys is well-known to increase the mobility of charge carriers (i.e., holes) in p-type FETs compared to conventional silicon-based FETs. The SiGe alloy system is one of few binary semiconductor alloys in which two constituent elements, silicon (Si) and germanium (Ge), form a perfectly miscible substitutional solid solution throughout the compositional range and simultaneously retain a random order with respect to atomic position. By varying compositions of the SiGe alloys, a wide range of strain (roughly 0-4%) can be established in the SiGe layer that allows enhancing device performance (e.g., higher on-state current) without the need for geometric scaling in the devices. However, for a random SiGe alloy, there is an increased carrier and phonon scattering due to the mass variation of constituent atoms in the lattice compared to an ordered SiGe alloy. The increased scattering reduces both charge carrier mobility and thermal conductivity, thus negatively impacting device performance. By imposing order (e.g., by forming SiGe digital alloy) to an otherwise random SiGe alloy during growth, scattering dues to the mass variance can be dramatically reduced.